Transmitter for multiplexed phase modulated singaling system



May 14, 1968 R. w. SANDERS 3,383,598

TRANSMITTER FOR MULTIPLEXED PHASE MODULATED SIGNALING SYSTEM Filed Feb.15, 1965 2 Sheets-Sheet 1 FIG.I

' 1 GAIN CONTROL SIGNAL SOURCE V (l0 l4 lfi fl? l8 Z BALANCED VARIABLE,,SUMMING PHASE POWER MODULATOR GAIN CIRCUIT MODULATOR AMPLIFIER [l6 LFIXED GAIN H I F I I I I I J RAY W. SANDERS INVENTOR ATTORNEY May 14,1968 R. W. SANDERS TRANSMITTER FOR MULTIPLEXED PHASE MODULATED SIGNALINGSYSTEM Filed Feb 15, 1965 2 Sheets$heet 2 30 s f FIG. 3 f2 FREQUENCYPHASE DOUBLER DETECTOR 2? 90 PHASE 32 SHIFTER ML PHASE 1 DETECTOR 26 33FREQUENCY DOUBLER ,25 3? VOLTAGE PlkOSVg CONTROLLED OSCILLATOR FILTER 3490 PHASE SHIFTER I l L PHASE J. DETECTOR 36 RAY W. SANDERS v IN VENTOR.

ATTORNEY United States Patent 3,383,598 TRANSMITTER FOR MULTIPLEXEDPHASE MDDULATED SIGNALING SYSTEM Ray W. Sanders, Los Angeles, Calif.,assignor to Space- General Corporation, El Monte, Calif a corporation ofCalifornia Continuation-impart of application Ser. No. 133,447, Aug. 23,1961. This application Feb. 15, 1965, Ser. No. 432,762

8 Claims. (Cl. 325-463) ABSTRACT OF THE DISCLOSURE This disclosurerelates to a multiplex phase modulation information transmission systemsuitable for transmitting four channels of binary data on a singlecarrier. The disclosure includes the unique transmitter which developscomposite waveforms from a pair of binary input channels and uses thiscomposite wave to phase modulate a carrier Through the use of a pair ofgain control circuits, one fixed and one variable, the signaldetectability of either of the two channels may be selectively enhancedas in the case where one channel has higher data rate than the other oris of greater significance.

The present invention relates in general to the communications art andmore particularly relates to a multiplexed system of signaling employingphase modulation, the transmitted information being uniquely reproducedby means of phase-locked circuits.

This is a continuation-in-part of the US. Patent application of Ray W.Sanders Ser. No. 133,447, filed Aug. 23, 1961, now US. Patent No.3,218,557.

Important information is oftentimes received or obtained from thetransmission of pulse trains. The present invention concerns itself withthe multiplexing of such pulse trains, that is, with the simultaneoustransmission of a pair of pulse trains on the same carrier wave. Inessence, a pair of pulse trains having different pulse repetition ratesare combined to form a single wave which varies between four differentvoltage levels. This last wave is then used to phase modulate a carrier,the phase shifts corresponding to the different voltage levels. At thereceiver site, the modulated carrier is processed to reproduce theoriginal pulse trains, phase-lock circuit theory being uniquely used inthe demodulate process.

Accordingly, one object of the present invention is to provide a systemof signaling involving the multiplexing process in connection with thetransmission of pulse trains.

Another object of this invention is to maximize detectability andreliability in multiplex transmission systems consistent with minimumtrans-mission power requirements.

Another object of the present invention is to provide a signaling systemin which phase-lock loop theory is significantly employed in thedemodulation process.

The novel features which are believed to be characteristic of theinvention, both as to its organization and method of operation, togetherwith further objects and advantages thereof, will be better understoodfrom the following description considered in connection with theaccompanying drawings in which an embodiment of the invention isillustrated by way of example. It is to be expressly understood,however, that the drawings are for the purpose of illustration anddescription only and are not intended as a definition of the limits ofthe invention.

FIG. 1 is a block diagram illustrating the transmitter portion of asignaling system according to the present invention;

FIG. 2 is a flow chart illustrating the voltage waveice forms existingat various points in the transmitter of FIG. 1;

FIG. 3 is a block diagram illustrating the receiver portion of asignaling system according to the present invention; and

FIG. 4 is a phase diagram of the signal from the transmitter of FIG. 1.

Referring now to the drawings and in particular to FIGS. 1 and 2therein, the transmitter is shown to include a balanced modulator 10whose two inputs are connected to receive a pair of pulse trains atinput terminals 11 and 12. The balanced modulator output is connectedthrough a variable gain device 14 to the first of two inputs to asumming circuit 15, the second input to the summing circuit beingcoupled through a fixed gain device 16 to input terminal 12. The summingcircuit output is then fed to a phase modulator 17 in preparation fortransmission, a power amplifier 18 being coupled between the phasemodulator and an antenna 20.

In operation, two pulse trains having different pulse repetition ratesare respectively applied to input terminals 11 and 12 and, therefore, tothe two inputs to balanced modulator 10, the pulses in these trainsprefer ably varying between 0 and 1 levels. Examples of the types ofpulse trains that may be applied to the balanced modulator are shown aswaveforms 21 and 22 in FIG. 2. As a result, the signal produced by themodulator is illustrated by waveform 23 in FIG. 2 and consists of apulse train whose pulses also vary between 0 and 1 values but whosepulse durations are unequal.

More specifically, in accordance with the principles governing balancedmodulators, signal 23 has a 1 value Whenever signals 21 and 22simultaneously or coincidentally have 1 or 0 values, that is to say,whenever they are simultaneously at the same voltage level. On the otherhand, signal 23 has a "0 value whenever signals 21 and 22 are atdifferent voltage levels, that is, whenever one of these two signals hasa 1 value and the other of them has a 0 value. The signals representedby waveforms 22 and 23 are applied to summing circuit 15 wherein theyare linearly added. However, before being applied to the summingcircuit, these signals are respectively passed through fixed andvariable gain devices 16 and 14 so that their amplitudes may be adjustedto different values for the purpose of providing four different voltagelevels in the signal produced at the output of the summing circuit andapplied to phase modulator 17. More particularly, a phase modulator is adevice that will shift the phase of a carrier, i.e., phase modulate acarrier, according to the level of the DC. voltage applied to it. Aswill be seen later, four different phase shifts are imposed upon acarrier in the embodiment being described, with the result that fourdifferent voltage levels must be applied to the phase modulator. Toachieve this result, waveform 23 is applied to variable gain device 14whose gain has been increased to sufficiently amplify signal 23 so that,when signal 23 is added to signal 22 in summing circuit 15, a waveformhaving the desired four different voltage levels, such as waveform 24,is produced.

The variable gain device may be controlled simply by an manual gaincontrol on an amplifier or may ,be remotely controlled as by a telemetrycontrol link or automatically through local feedback control. Each ofthese devices are represented in the drawing as the gain control signalsource 19 coupled tothe variable gain device 14. The introduction of theselective gain relationship between the output of balanced modulator 10and the signal on input terminal 12 allows the selective control ofrelative power level of the two information channels as hereinafterdescribed. An example of the kind of waveform that may be obtained atthe output of the summing circuit and that may be applied to the phasemodulator is illustrated by waveform 24 in FIG. 2.

In phase modulator 17 a carrier is subjected to any one of four possiblephase shifts, the particular phase shift experienced by the carrier atany one time depending upon the voltage level of signal 24 at that time.By way of example, the four phase shift angles may be Thus, again by wayof example, the carrier out of the phase modulator may at any time beany one of the following, namely:

A sin (ml-l-E-I-oz) A sin (wi+ga) and A sin (wt--ot) Following the stepof phase modulation, the carrier signal is power amplified in circuit 18and thereafter applied to antenna 20 for radiation into space.

The phase modulated signals applied to the antenna 20 are presented inthe phase diagram of FIG. 4 showing four vectors designated 1, 2, 3 and4 corresponding to the four discrete voltage levels from the summingcircuit 15. The fixed gain device 16 is adjusted so that the twodiscrete voltage levels on terminal 12, as amplified in the fixed gaindevice 16, produce a 1r/2 and phase shift, respectively, in the phasemodulator 17 output. The gain level of device 14 on the other hand isset to produce discrete voltage levels corresponding to phase shift of adegrees where is preferably falls between and 90. The value of a is notarbitrarily established but rather is either controlled in advance orduring transmission as a function of the relative data rates of the twochannels or alternately of the reliability requirements of the twochannels denoted by the terminals 11 and 12.

As shown in FIG. 4 a is less than 45 and obtained by setting the gainlevel of device 14 at less than that of fixed gain device 16. Thischoice of a is preferred Where the data rate of information at terminal12 exceeds the data rate from the source connected to terminal 12. Whereon is less than 45, the component power radiated from the transmitterantenna 20 carrying the information on terminal 12 is represented by thevertical component (2a) of the phase diagram of FIG. 4. The transmittedpower of signal 21 at terminal 11 is proportional to the component(21)). In terms of phase shift a signal on terminal 11 produces phaseshift deviations of 20: upon each transition while a signal on terminal12 produces phase shifts of 1r20t degrees. Since the angle 7r2a isgreater than 20: by definition, the detection probability of theinformation from terminal 12 is enhanced. In a situation as hereindescribed, this enhanced detection probability is designed to offset thereduced probability of error-free reception at higher data rates. In thecase where the data rates are substantially equal and the transmissionreliability of one channel is necessarily higher, the high reliabilitychannel signal should be applied to terminal 12 and a gain reducingsignal from source 19 be applied to variable gain device 14 to reducethe incremental voltage output to the summing circuit 15 and the valueof a. Therefore, this invention is particularly suited for multiplexdigital transmission systems having different data rates and equalreliability standards or substantially equal data rates and higherreliability requirements on one data source.

Reference is now made to the receiver portion of the system in FIG. 3,which is shown to include a voltage controlled oscillator 25 that isconnected through a frequency doubler 26 and thereafter through a phaseshifter 27 to the first of two inputs to a phase detector 28. Afrequency doubler 30 is also connected between the first input to phasedetector 28 and the input to the receiver, namely, an input terminal 31.In addition to being connected to frequency doubler 26, voltagecontrolled oscillator 25 is also connected to the first of two inputs toa phase detector 32 whose other input is connected to input terminal 31,the output of this phase detector being one of the two outputs for thereceiver and is designated 33. Again, voltage controlled oscillator 25is coupled through a 90 phase shifter 34 to still another phase detectorcircuit 35, this last detector circuit also being connected betweeninput terminal 31 and the second output for the receiver which isdesignated 36. Finally, a suitable low pass filter 37 is coupled betweenthe output end of phase detector 28 and the input end of oscillator 25.

In considering the operation, it is briefly stated that transmissionsare received at input terminal 31 and, after being processed, waveforms21 and 22 generated at the transmitter site are reproduced at outputterminals 33 and 36 respectively. More particularly, upon receipt, thetransmitted phase modulated carrier is applied to frequency doubler 30,with the obvious result that the signal applied to phase detector 28 isat twice the frequency of the incoming signal. Furthermore, due to thefrequency doubling step, the four phase shift angles are now:

+1r+2u +1r2ot --1r+2ot and 1r2ot At the same time, the signal out ofvoltage controlled oscillator 25, which signal is at the same frequencyas the transmitted carrier, is applied to frequency doubler 26 and thento 90 phase shifter 27 wherein, as is implied, the frequency of theoscillator signal is doubled and its phase shifted by 90 before beingapplied to phase detector 28. Under conditions where the carrierreceived at input terminal 31 is unmodulated, the carrier and oscillatorsignals not only have the same frequency but are quickly brought intophase with each other, with the result that the two signals applied tophase detector 28 are at the same frequency but out of phase with eachother by an angle of 1r/2 degrees.

Since a phase detector circuit produces a maximum output voltage whenthe signals applied to it are in phase and a zero output when thesignals applied to it are 17/ 2 degrees out of phase, the output fromphase detector 28 would be substantially zero under the conditionsmentioned. However, where the received carrier is phase modulated asherein, the two signals applied to phase detector 28 have the samefrequency but are out of phase with each other by the angles degrees. Itwill, therefore, be obvious to those skilled in the art that the signalout of phase detector 28 and passed through low pass filter 37 willfluctuate between two limits corresponding to the It will also berecognized that in response to the signal out of low pass filter 37 thatthe phase of the voltage controlled oscillator signal willcorrespondingly be shifted ia degrees. In other words, the signalapplied to phase detector 32 and to phase shifter 34 is constantly beingphase shifted by in: degrees. Since the phase modulated carrier is alsoapplied to phase detector 32, the signal out of the detector and finallyreproduced at output terminal 33 is that of waveform 21 in FIG. 2, whichwaveform was instrumental in producing the in phase shifts at thetransmitter site.

The transmitted carrier is also applied to phase detector 35 to which isfurther applied the output from voltage controlled oscillator 25 afterthat output has been passed through 90 phase shifter 34. Since theoscillator signal has previously experienced :a phase shifts, it will berecognized that the two signals applied to phase detector 35 thereforetend to be in phase with each other, with the result that the phasedetector output signal reproduced at output terminal 36 is substantiallya duplicate of waveform 22 in FIG. 2, which waveform was used to producethe i1r/2 phase shifts in the carrier at the transmitter site. It isthus seen that information signals 21 and 22 applied to the transmitterare reproduced by the receiver.

Having thus described the invention, what is claimed as new is:

1. In a signaling system a transmitter for transmitting a particularphase modulated carrier in response to the application thereto of firstand second pulse trains, said transmitter comprising:

means receptive of the first and second pulse trains and operable inresponse thereto to produce a third pulse train whose amplitude variesbetween four different voltage levels; said last means, including afirst combiner, for producing a first set of two of the four differentvoltage levels derived from one of the pulse trains and said last meansincluding a second combiner for producing second set of two voltagelevels derived from a composite of the first and second pulse trains;

means for adjusting the relative level of the two sets of differentvoltage levels with respect to each other;

means for phase modulating a carrier as a function of the magnitude ofthe four different voltage levels; and

means for radiating the phase modulated carrier wave.

2. In a signaling system, a transmitter for radiating a multiplexedphase modulated carrier comprising:

means receptive of first and second pulse trains and operable inresponse thereto to produce a third pulse train constituting a compositeof the first and second pulse trains;

means combining the third pulse train with one of said first pulsetrains to produce a fourth pulse train having four discrete levels;

means for phase modulating a carrier with the fourth pulse train; and

means for radiating the phase modulated carrier.

3. The combination in accordance with claim 2 including means forselectively varying the level of the third pulse train with respect tosaid first pulse train.

4. The transmitter defined in claim 2 wherein said combining meansincludes a summing circuit for linearly adding signals applied to it, afixed gain device coupled to apply the second pulse train with apredetermined amplitude to said summing circuit, and a variable gaindevice coupled between said balanced modulator and said summing circuitfor adjusting the amplitude of said third pulse train in such a mannerthat said fourth pulse train is produced by said summing circuit.

5. A transmitter for transmitting phase modulated multiplexed pulsetrains comprising:

input terminal means for receiving respective first and second pulsetrains;

means for combining the first and second pulse trains into a third pulsetrain having two discrete levels, one indicative of coincidence of thefirst and second wave trains and the second indicative of lack ofcoincidence thereof;

means for independently controlling the gain of the third pulse trainwith respect to the first pulse train;

means for combining the first and third pulse trains to produce a fourthpulse train having a plurality of levels representing the combined pulsetrains;

means for phase modulating a carrier with the fourth pulse train; and

means for radiating the phase modulated carrier.

6. A transmitter in accordance with claim 5 wherein said first andsecond pulse train combining means comprises a balanced modulator.

7. A transmitter in accordance with claim 5 wherein said first and thirdpulse train combining means comprises a summing circuit.

8. In a signaling system a transmitter for transmitting a particularphase modulated carrier in response to the application thereto of firstand second pulse trains, said transmitter comprising:

a balanced modulator receptive of the pulse trains and operable inresponse thereto to produce a third pulse train whose pulses occurwhenever the pulses of the first and second pulse train coincide intime;

means for adding the second and third pulse trains to produce a fourthpulse train, said means including circuits for adjusting the amplitudesof said second and third pulse trains in such a manner that said fourthpulse train varies between four different voltage levels; and

output apparatus for radiating a phase modulated carrier signal, saidapparatus being coupled to receive said fourth pulse train and operablein response thereto to produce carrier phase shifts of 9/1959 Doelz etal 325-39 X 8/1962 Buff 17866 X ROBERT L. GRIFFIN, Primary Examiner.

JOHN W. CALDWELL, Examiner.

W. S. FROMMER, Assistant Examiner.

